Conventionally, thin film structures having silicon (Si) and such as a main ingredient have been used in semiconductor layers for forming thin film transistor channel layers that function as switching elements for an image display apparatus using liquid crystals. A thin film structure using amorphous silicon, which does not exhibit crystallinity, is commonly used. This is because an amorphous silicon thin film can be formed using a low temperature process with relative ease, and the manufacturing cost can be reduced. However, an amorphous silicon thin film has a low mobility of 0.6 cm2/Vs, which presents a problem.
An image display apparatus using liquid crystals has a structure for supplying an electric charge via a switching element to a pixel electrode that corresponds to a pixel. For an image display apparatus with a large number of pixels, a thin film transistor having a channel layer with high mobility is required to improve the switching rate.
As one example of a film structure other than the amorphous silicon thin film, a structure using a microcrystalline thin film has been proposed. A film forming method using plasma chemical vapor deposition, hereafter referred to as “the plasma CVD method”, forms a film of silicon in a microcrystalline state.
The plasma CVD method applies a high frequency electric field on a source gas of the semiconductor phase to cause collisions between the source gas and electrons so as to turn the source gas into a reactive plasma state before forming a film on a substrate. When a conventional film forming method is employed, the electric field intensity density irradiated on the source gas needs to be controlled at or below a prescribed value. This is because, when a high energy electric field is applied, the SiH2 produced by breaking down SiH4 is very reactive and bonds to itself to form a polymer before reaching the substrate surface. For this reason, when using the plasma CVD method, a low intensity electric field is irradiated on the source gas SiH4 to break it down to a relatively less reactive SiH3 before forming a film. However, when such a technique is used, a high ratio of dangling bonds of the silicon atoms that constitute the microcrystalline thin film terminate with hydrogen, which makes it difficult to obtain a microcrystalline thin film with high mobility.
Another method of forming a microcrystalline thin film involves the following: after depositing amorphous silicon on a substrate with the plasma CVD method, hydrogen ions or hydrogen plasma is irradiated at the amorphous silicon thin film. According to this method, a source gas prepared by mixing SiH4 and hydrogen is used to form approximately 15 nm of amorphous silicon film on the substrate surface by means of the plasma CVD method, followed by introduction of just hydrogen, and then hydrogen discharge is carried out. Such hydrogen discharge microcrystallizes the amorphous silicon film and converts it to a microcrystalline silicon film.
However, the conventional method for obtaining microcrystalline silicon film from amorphous silicon film has various problems and achievement of sufficient mobility is known to be difficult. The problems of conventional technology are discussed below.
First of all, the aforementioned film forming method has a problem in that the crystalline structure of the obtained microcrystalline silicon film is not good. In the aforementioned film forming method, amorphous silicon film is formed once and then its crystalline structure is converted to microcrystals; however, converting the crystalline structure of the entire amorphous silicon film is not easy and there is a high probability of a certain percentage of amorphous silicon film remaining. Since amorphous silicon film has low mobility, the carrier mobility will be reduced, depending on how much amorphous silicon film remains.
Also, dangling bonds of silicon atoms contained in the amorphous silicon film are terminated with hydrogen atoms. When a hydrogen discharge is used to obtain microcrystalline thin film, hydrogen dissociates in some of the dangling bonds of the silicon atoms and such dangling bonds will bond with other silicon atoms to form microcrystals. To increase bonding between silicon atoms, the intensity of the hydrogen discharge needs to be sufficient; however the use of a high intensity hydrogen discharge in the aforementioned film forming method results in damaging crystalline structures other than the amorphous silicon film, such as the crystalline structure of the substrate. Therefore, when the aforementioned film forming method is used, hydrogen discharge with sufficient intensity cannot be applied, leaving many dangling bonds still terminated with hydrogen, which makes it difficult to improve the mobility of the microcrystalline thin film.